well - you have seen the effects of the oil on the rubber parts, It would be in your best interest to replace all the rubber pipes and radiator cap, if they started to delaminate, they can suddenly burst when under pressure, which can lead to severe overheating.
to clean the engine internally and if you cannot find any other soap - after running it for such long on the lemon max soap, remove a heater pipe and shove your garden pipe in it (make sure the engine is dead cold) let the water flow in the engine and drain out the radiator lower pipe or the radiator drain petcock for about 10 minutes, back flush the heater radiator too, shove the pipe into the heater connection to let it flush out.
to finish with very clean water you can get a water cooler or get a bucket and attach a valve to somewhere on the bottom (like a water cooler) - use a pipe to connect it to the heater connection again, fill the bucket with a 5 gallon container of nestle water or distilled water, open the valve and let gravity flush out the water.
this coolant has a service life of 3 years or 50,000 kms - in your engine the problem is getting water out of the block, you will need to use compressed air to blow it out. In such engine type I flush with water and then distilled then add concentrated coolant at exactly half the quantity of the system then top up with water, this way you will always get 50% conc. coolant in there.
the bottle you are showing is already 60% water, filling such requires you to remove all water from the engine then fill it up. if you leave water in the engine it will dilute it even further.
to remove the thermostat cover bolt, and if its stuck then you need to douse it with a penetrating oil like atf and acetone or WD40, you may need to apply some heat too. I would suggest to replace the thermostat too as its rubber seal will also have been destroyed by the oil.
